Field of the Invention
This invention relates to a nonaqueous electrolyte secondary battery and a method for manufacturing a nonaqueous electrolyte secondary battery, and particularly relates to a nonaqueous electrolyte secondary battery improved in characteristics after continuous charging and a method for manufacturing such a nonaqueous electrolyte secondary battery.
Description of Related Arts
In recent years, size and weight reduction of mobile information terminals, such as cellular phones, notebook computers and PDAs, has rapidly progressed. Batteries used as their driving power sources are being required to achieve a higher capacity. To meet such a request, nonaqueous electrolyte secondary batteries using a nonaqueous electrolytic solution to perform charge and discharge by moving lithium ions between positive and negative electrodes are widely utilized as new secondary batteries having high output and high energy density.
In these nonaqueous electrolyte secondary batteries, materials commonly used as positive-electrode active materials include lithium cobalt oxide (LiCoO2), spinel lithium manganate (LiMn2O4), cobalt-nickel-manganese-containing lithium composite oxides, and aluminum-nickel-cobalt-containing lithium composite oxides. On the other hand, materials used as negative-electrode active materials include carbon materials, such as graphite, and materials capable of forming an alloy with lithium, such as Si and Sn.
However, in more recent years, mobile information terminals have enhanced their entertainment features including a video playing feature and a gaming feature and have thereby tended to further increase the power consumption. Therefore, nonaqueous electrolyte secondary batteries are being required to achieve a still higher capacity.
Possible measures for increasing the capacity of a nonaqueous electrolyte secondary battery include (1) increasing the capacity of the active material, (2) increasing the charge voltage, and (3) increasing the amount of active material packed, i.e., increasing the packing density.
Particularly if the charge voltage is increased, there arises a problem of ease of decomposition of the electrolytic solution. More particularly, if the battery is stored or continuously charged at high temperatures, the electrolytic solution may decompose to produce gas, thereby causing problems of swelling of the battery and increased internal pressure of the battery.
Published Japanese Patent Application No. 2007-538365 proposes a lithium secondary battery using a nitrile group-containing compound for an electrolytic solution, wherein the positive-electrode active material used is a lithium-containing composite oxide which contains one or more elements selected from the group consisting of alkali metals, alkaline earth metals, 13th group elements, 14th group elements, 15th group elements, transition metals and rare earth elements and is doped with a heterogeneous metal selected from Al, Mg, Zr, Fe, Zn, Ga, Sn, Si and Ge. The document describes that thus the nitrile groups bind to the surface of the positive electrode at high temperatures to form a complex, and the complex serves as a protective film for blocking active sites on the surface of the positive electrode. The document also describes that the protective film inhibits part of a transition metal from being eluted from the positive-electrode active material and deposited on the negative electrode and inhibits the occurrence of a side reaction and gas generation due to reactions between the electrolytic solution and the positive electrode, whereby lithium can be smoothly storaged and released even at high temperatures to thereby inhibit the deterioration in lifetime characteristic.
Published Japanese Patent Application No. 2008-108586 proposes that an oxide of a lithium-containing transition metal containing at least one metal element selected from Mg, Ti, Zr, Ge, Nb, Al and Sn is used as a positive-electrode active material, and a compound having two or more nitrile groups in the molecule is contained in the electrolytic solution. The document describes that the nitrile compound has the function of forming a coating on the surface of the positive-electrode active material, and the formation of such a surface protective coating prevents direct contact of the electrolytic solution with the positive-electrode active material to inhibit gas generation, whereby the crystal structure of the positive-electrode active material can be stabilized to increase the storage performance and safety of the battery.
Published Japanese Patent Application No. 2002-279991 proposes to use a positive-electrode active material in which two or more compound layers each made of a hydroxide, an oxyhydroxide, an oxycarbonate or a hydroxycarbonate of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As or Zr are formed on the core of the positive-electrode active material. The document describes that thus the cycle characteristics are improved.
These conventional techniques, however, do not describe any approach for reducing the drop in discharge voltage after storage at high temperatures or after continuous charging at high temperatures.
Meanwhile, Published Japanese Patent Application No. 2008-226495 describes a means for impregnating particulate powder of a lithium-containing composite oxide (positive-electrode active material) with a solution (impregnating solution) containing a lanthanum source by spraying. The document also describes that the amount of impregnating solution is preferably controlled within the range from 0.1% to 80% by weight relative to the weight of a matrix used, more preferably within the range from 1% to 75% by weight, and particularly preferably within the range from 1% to 40% by weight. In other words, the method described in Published Japanese Patent Application No. 2008-226495 shows that the amount of impregnating solution may be within the range from 40% to 80% by weight. Therefore, according to this method, the impregnating solution can be sprayed to such an extent that the positive-electrode active material is soaked therein.